What the sources claim for it
Limit and constraint. The authority under which something is bound rather than granted.
History
Where does Saturn come from?
Saturn sits at the outer edge of the visible system and moves slowest — roughly thirty years to the circuit — and the whole of its symbolic load follows from that. It takes lead, the heaviest and dullest of the seven metals, and Saturday, and in the planetary correspondences it governs limits, endings, imprisonment and long time.
In the Key of Solomon the pentacles of Saturn are the ones used for constraint and for operations touching the dead, and they are to be made in lead, on a Saturday, in Saturn's hour. A reader who knows the system reads all of that off the single sign.
1 figure
Who is it attached to?
Figures who carry it
An attribute rather than a signature — held, worn or associated in the texts and the images.
Afterwards
What did it turn into?
Retained unchanged as the astronomical symbol for the planet, and still the alchemical sign for lead in historical chemistry.
Which texts describe it?
Oldest first, with what each one does with the mark. A mark described in a Bronze Age incantation and a mark first drawn in a seventeenth-century manuscript are different kinds of claim, whatever the shape has in common.
Renaissance & Early-Modern Occult Angelology
Agrippa, Barrett, and their compilers · 16th-19th c. · Latin and English
Agrippa's tables tie the sign to its metal, square, day and hours.
Read Renaissance & Early-Modern Occult Angelology in full ↗Cover: Public domain · Otto Rietmann · Wikimedia file page
The Key of Solomon (Clavicula Salomonis)
Anonymous; ed. S. L. MacGregor Mathers · 15th-16th c. · Latin and Italian
Seven pentacles of Saturn, in lead, for binding and for the dead.
Read The Key of Solomon (Clavicula Salomonis) in full ↗Cover: Public domain · Unknown author · Wikimedia file page
Nearby
Related marks
Jupiter
A stylised Greek zeta for Zeus. The sign of increase, favour and judgment in one's favour — the grimoires' preferred authority for asking.
Mars
Shield and spear, iron, Tuesday — and, since Linnaeus, the biological sign for male, which is a much later and quite separate job.
The Sun
A circle with a point at its centre — the oldest form in the set, and the one the alchemists used for gold without further explanation.
Venus
Copper, Friday, and every operation to do with love and favour — the counterpart to Mars in the grimoires long before it was in biology.
Mercury
A caduceus drawn as a glyph — the one sign in the set that names a metal, a planet and a god that all behave the same way.
The Moon
A crescent, silver, Monday — the planetary authority for anything involving dreams, water, change and things done at night.